Polyester films have been used in various application fields because they are excellent in transparency, dimensional stability, mechanical properties, heat resistance, electrical properties, etc.
In particular, in recent years, the polyester films have been frequently used as a base material for transparent conductive laminates having an increasing demand for touch panels, in place of glass. In the transparent conductive laminates, an ITO (indium tin oxide) film is formed on a biaxially oriented polyester film as a base material directly or through an anchor layer by a sputtering method. Such a biaxially oriented polyester film is generally subjected to heat-processing.
For example, the biaxially oriented polyester film is allowed to stand at 150° C. for 1 hr to subject the film to low-temperature shrinking (Patent Document 1). Also, the biaxially oriented polyester film is subjected to treatments such as heat treatment at 150° C. for crystallization of ITO thereon (Patent Document 2).
However, the polyester film has such a problem that when subjected to such a high-temperature treatment for a long period of time, oligomers (low-molecular weight components of the polyester, in particular, an ester cyclic trimer thereof) included in the film are deposited and crystallized on a surface of the film, so that deterioration in visibility of the film owing to a whitened appearance thereof, defects in post-treatments, contamination of a process or members present in the process, etc., tend to occur. For these reasons, the transparent conductive laminates using the polyester film as a base material tend to hardly exhibit satisfactory properties.
As the measure for preventing deposition of the above oligomers, there has been proposed, for example, the method in which a cured resin layer comprising a crosslinked product of a silicone resin and an isocyanate-based resin is formed on a polyester film (Patent Document 3). However, since the cured resin layer of the polyester film is formed by heat-curing, it is required to subject the film to a high-temperature treatment in order to dissociate a blocking agent for the isocyanate-based resin. Thus, in the method, there tends to occur curling or sagging of the film in the course of processing, and therefore careful attention must be paid upon handling the film.
In consequence, when a coating layer is formed on the polyester film to take a measure for reducing an amount of oligomers deposited thereon, it is required that the coating layer thus formed has a still higher heat resistance than that of conventional ones, and also exhibits a good performance of preventing deposition of oligomers by itself.
Furthermore, the polyester films used, for example, for touch panels tend to be frequently subjected to hard coating process in order to enhance anti-curling property and scratch resistance of the film and improve properties such as surface hardness. For this reason, there is an increasing demand for polyester films comprising a coating layer that is excellent in adhesion property to the hard coat.
Also, there is known a transparent conductive film comprising a laminate constituted of two polyester films bonded to each other and a transparent electrode pattern formed on the laminate. When the transparent conductive film is used in resistive membrane touch panels, since the two polyester films therein are bonded to each other through an adhesive layer having a cushioning property, the obtained touch panels are enhanced in pen-inputting fastness and surface pressure fastness (Patent Literature 4). In addition, when the transparent conductive film is used in capacitance touch panels, the two polyester films can be bonded to each other through a cured adhesive layer. In such a case, it is possible to enhance touch sensitivity of the resulting touch panels and suppress undulation thereon (Patent Literature 5). For these reasons, it is presently necessary to provide polyester films comprising a coating layer that is excellent in adhesion property to the adhesive layer and the cured adhesive layer.
However, the conventional polyester films have failed to satisfy both a measure for reducing an amount of oligomers deposited thereon and good adhesion property to various functional layers to a sufficient extent.